Does that board have Vista drivers available for it? I did some quick looking around and noticed quite a few complaints on-line about not having vista drivers (folks were using XP drivers in place of it).
When I went to Nvidia's site, I they didn't seem to have a driver for the nforce3 systems - just Nforce Pro 3000 (I don't think they are the same thing).
You might be out of luck - I hope I'm wrong.
Now one thing to note about this problem for me, is if I shut down my system, then powered off the PSU, then turned everything back on and restarted the system, the NICs showed back up. Can you try the same thing?